This invention relates generally to communication systems and, more particularly, to a technique which minimizes the transmitted peak power levels in an orthogonal frequency division multiplexed (OFDM) waveform by randomizing the digital data being communicated.
The frequency tones transmitted in an OFDM system are purposely spaced so that the frequency components of the tones are non-additive. That is, multiple tones can be received in a wideband receiver without a first tone canceling out or adding to the amplitude of neighboring tones. This permits the communication system to transmit and receive tones simultaneously, without mutual interference. However, these orthogonal frequency components still interact (are additive) with each other from the perspective of time domain analysis.
In the exemplary OFDM system described in Provisional Application, Ser. No. 60/140,648, many orthogonal frequency tones are transmitted or received simultaneously. These tone frequency sums are ultimately communicated by base stations and remote units through a wired or radio frequency (RF) link using power amplifiers and receivers. At any one instant, the waveform being transmitted by a power amplifier is a voltage which is a sum of the voltages of the tones. If the phase relationship between tones is random, then the peak voltage falls within a well defined range. For example, with 288 tones of equal magnitude, but random phases, the peak voltage in the time domain OFDM waveform is distributed in the range of 8 dB to 14 dB above the RMS voltage level, but may be as high as 27 dB above it when the phases align. This large peak voltage is known as “cresting” of the OFDM waveform.
The OFDM system requires a high degree of linearity in the communication of the OFDM tones. Non-linear transmission or reception of a first signal generates harmonic and spurious products which have a frequency relationship to the first signal. In a system that depends on frequency orthogonality, the non-linear communication of tones will generate tone products which act to destroy this relationship of orthogonality. Alternately stated, in an OFDM system which relies upon the amplitude components of the tones to relay information, it is critical that the amplitude information is not degraded in the amplification process. Thus, in an OFDM system it is necessary to use linear power amplifiers to transmit the OFDM waveform.
As is well known in the art, the bias on these types of amplifiers (Class A) must be set especially high. Class A amplifiers draw a great deal of current regardless of the RMS value of the actual power being transmitted. The amount of power drawn by a Class A amplifier is proportional to the peak voltage that the amplifier must be capable of amplifying. It is therefore desirable to keep expected peak waveform voltage to a minimum. This permits a linear amplifier with a minimum dynamic range to be used, without clipping the OFDM waveform and thus causing non-linearities. This also minimizes the power consumption of the hardware, which extends the life of the equipment, and in the case of battery powered devices, such as remote units, extends the life of the batteries.
These same problems are a concern in the design of the receiver. Although receivers do not generally consume a great deal of power, as compared to the power amplifier, dynamic range is critical. The receiver must have a dynamic range large enough to recover a transmitted signal at a large range of input power levels, where the strength of the received signal corresponds to the distance between the receiver and transmitter.
In the OFDM system, digital data is modulated using QAM, QPSK, PSK, or other schemes, as are well known in the art. These modulation systems rely on the amplitude and/or phase of a symbol to convey information. Ultimately, the phase and/or amplitude of the OFDM tone is used for communication of information. In voice communications, and in many data communications, the information and, therefore, the patterns of digital data representing the information are random. The random pattern of digital data translates through the modulation process into a random selection of tone phases and amplitudes. When this random collection of OFDM tone frequencies is transmitted simultaneously, it is statistically unlikely that they will add to create a very large peak in the time domain OFDM signal. In this situation, there is no need to be concerned with saturating an OFDM power amplifier designed to operate in this scenario. Saturation of the power amplifier due to a large peak voltage is more likely to occur when the digital data to be communicated is highly correlated or the same, for example, when information is being sent that represents an image with a highly uniform background. Then, the data consists of a long pattern of “0”s or “1”s. In this circumstance, where the digital data forms a repetitious pattern, it is likely that the OFDM tones will have a highly correlated phase and amplitude relationship. This correlation of the phase leads to a large peak in the OFDM waveform. In order to handle this large peak without clipping the OFDM waveform, the power amplifier must operate in a larger dynamic range than the range resulting from a random combination of tone amplitude and phases. Operation in a large dynamic range results in the power amplifier consuming more power.
It would be advantageous if the peak power levels required for transmission in an OFDM communication system could be kept to a minimum to preserve linearity, reduce power consumption, and simplify the design of the transmitter and receiver.
It would be advantageous if the dynamic range of the power levels required for transmission in an OFDM communication system could be minimized to avoid clipping in the power amplifier and to reduce power consumption.